1. Field of the Invention
The present relates to a system for making an on-line, non-destructive and continuous determination of degree of alloying in galvannealed steel sheets with the use of X-ray diffractometry, said galvannealed steel sheets being produced by the application of heat treatments just following zinc coating.
2. Statement of the Prior Art
So far, galvannealed steel sheets--to which paintability, paint adhesion property and weldability are imparted in addition to the corrosion resistance of galvanized steel--have been produced with wide applications. This galvannealed steel sheets has been produced by subjecting steel sheets to a continuous process of hot-dip galvanizing, electro-galvanizing or zinc vapor depositing and then to post-heat treatments, thereby alloying the zinc coating and the base steel each other.
When the steel sheets are heat-treated after zinc coating in such a way as mentioned above, the .eta.-Zn phase in coating layer is disappeared with the progress of alloying by diffusion of Fe and Zn, and the .zeta. (FeZn.sub.13), .delta..sub.1 (FeZn.sub.7) and .GAMMA. (Fe.sub.5 Zn.sub.21) phases grow successively.
Heretofore, it has been said that the quality of the galvannealed steel sheets have a close correlation with the degree of alloying. When the degree of alloying is so low that a relatively soft, thick layer of the .zeta. phase remains on the surface of the coating layer, it is so increased in the surface friction with a mold (die) during press forming that its feeding into the mold gets worse, it is preferable to reduce the proportion of the .zeta. phase. In view of the paintability and cosmetic corrosion resistance of galvannealed coating thereon, however, the proportion of the .zeta. phase should preferably remained. When the degree of alloying is so high that a hard but brittle, thick layer of the .GAMMA. phase grows between the coating layer and the base steel on the other hand, "powdering"--a phenomenon that the coating layer peels off the .GAMMA. phase in powdery forms--takes place. If this powdering phenomenon occurs to some considerable extent, not only is press forming adversely affected, but the corrosion resistance of the galvannealed steel sheets become poor, because the coating layer disappears substantially, if not completely.
In order to produce an galvannealed steel sheets having improved quality, it is thus required to control the degree of alloying, whereby the growth of the .GAMMA. phase is controlled to permit an appropriate proportion of the .zeta. phase to remain on the surface of the coating layer.
For determining of the degree of alloying of galvannealed steel sheets, various methods have been used so far in the art, as mentioned below.
The simplest method of all involves a visual or photometric determination of a hue change in the surface of the coating layer just following galvannealing or a visual determination of the amount of the coating layer peeled from a sample by a bending/bending-back test--the so-called powdering test, but it is inaccuracy. There is also available a chemical analysis of the average content of Fe in the coating layer of a test piece, which has conventionally been used as an index to the degree of alloying. However, a problem with this chemical determination is that it takes much time from sampling to the completion of analysis, incurring some time lag in feeding the output back to heat-treating in alloying furnace.
In order to determine the degree of alloying from the structure of a coating layer, to this end, a sample may be polished in cross-section to observe the cross-section of the coating layer under an optical microscope or a scanning electron microscope etc, thereby measuring the thicknesses of the .zeta., .delta..sub.1 and .GAMMA. phases. Alternatively, a sample may be measured for its X-ray diffraction profiles with an analytical X-ray diffractometry device, whereby the degree of alloying is determined with the X-ray diffraction intensities of the .zeta., .delta..sub.1 and .GAMMA. phases. These methods are preferable in a sense of providing a determination of the degree of alloying from the structure of the coating layer. However, a problem in these methods is that it takes much time from sampling to the completion of analysis, incurring some time lag in feeding the output back to heat-treating in alloying furnace. A problem common to all the above-mentioned powdering test, chemical analysis, cross-sectional observation and X-ray diffractometric analysis is that because of their destructive examination, it is impossible to provide a determination of the degree of alloying of galvannealed steel sheets along the entire widthwise or line direction.
On the other hand, X-ray diffractometric methods have been proposed to make an on-line, non-destructive and continuous determination of the degree of alloying. For instance, Japanese Patent Laid-Open No. 61-148355 discloses a method for determining the average content of Fe in a coating layer by measuring the X-ray diffraction intensities of the .GAMMA. phase with a interplanar spacing of d=approx. 1.22 .ANG. and the .alpha.-Fe phase with a interplanar spacing of d=approx. 1.44 .ANG. and substituting the two measurements for a functional equation for the average content of Fe in the coating layer, using as the variables the pre-calculated X-ray diffraction intensities of the .alpha.-Fe and .GAMMA. phases. With this method in which what is measured is limited to the .GAMMA. phase, it is impossible to measure the amounts of the .zeta. and .delta..sub.1 phases formed and find the structure of the coating layer accurately.
With these conventional methods for determining the degree of alloying by X-ray diffractometry, it is also impossible to measure X-ray diffraction intensities accurately, because the detector for diffracted X-rays is fixed at a 2.theta. position (diffraction angle) of a specific lattice plane of the Fe-Zn intermetallic compound phases to be measured. In other words, the .zeta., .delta..sub.1 and .GAMMA. phases are all non-stoichiometric compounds that vary in the content of Fe depending upon the degrees of diffusion of Fe and Zn, as can be seen from an equilibrium state phase diagram on which the .zeta. phase has an Fe content range of about 5.5 wt. % to about 6.2 wt. %, the .delta..sub.1 phase has an Fe content range of about 7.0 wt. % to 11.4 wt. % and the .GAMMA. phase has a Fe content range of about 20.0 wt. % to about 28.0 wt. %. Depending upon the degree of alloying, there are thus changes in the interplanar spacing of the .zeta., .delta..sub.1 and .GAMMA. phases and consequently results in the change of the 2.theta. positions of the X-ray diffraction peaks of the respective phases. This phenomenon--which has been experimentally confirmed by inventors of the present invention--will now be explained more illustratively with reference to FIG. 1 showing the X-ray diffraction profiles of galvannealed coating, each with a coating weight of about 45 g/m.sup.2, which have been heat-treated in salt bath at 500.degree. c. for 5 seconds and 60 seconds for alloying, with the use of a Cr tube (operating at a tube voltage of 40 kV and a tube current of 70 mA). In FIG. 1, reference numeral 1 represents for the X-ray diffraction profile of the galvannealed coating prepared by a 5-second heat-treatment at 500.degree. c. and 2 stands for that of the galvannealed coating prepared by a 60-second heat-treatment at 500.degree. c. As can be understood from FIG. 1, the apex positions of the X-ray diffraction peaks of the .zeta., .delta..sub.1 and .GAMMA. phases of the galvannealed coating heated at 500.degree. c. for 5 seconds lie at 2.theta.=130.0.degree., 2.theta.=126.0.degree. and 2.theta.=139.0.degree., respectively, but those of the galvannealed coating having an increased degree of alloying because of having been heated at 500.degree. c. for 60 seconds are found at somewhat higher levels, say, 2.theta.=130.5.degree., 2.theta.=127.0.degree. and 2.theta.=139.5.degree., respectively. As an example, now assuming that the degree of alloying is measured at the X-ray diffraction intensity of the .delta..sub.1 phase having a interplanar spacing of d=approx. 1.28 .ANG., it is then noted that with a diffracted X-ray detector fixed at the apex position of the X-ray diffraction peak of the .delta..sub.1 phase of the galvannealed coating heated at 500.degree. c. for 5 seconds, say, 2.theta.=126.0.degree., the X-ray diffraction intensity of the .delta..sub.1 phase of the galvannealed coating--having an increased degree of alloying because of having been heated at 500.degree. c. for 60 seconds--is measured to be about 14,000 (c.p.s.) at this position. In other words, the measurement is in error by as much as about 8,000 (c.p.s.), because the peak apex position of the latter galvannealed coating lies at 1.theta.=127.0.degree. where the X-ray diffraction intensity is about 22,000 (c.p.s.). Therefore, when the diffracted X-ray detector is fixedly located at the 2.theta. position of a specific lattice plane, the peak apex position changes with a change in the degree of alloying, deviating from that detector and so rendering it unable to measure the X-ray diffraction intensity, i.e. the degree of alloying, constantly and normally.
As already stated, the .zeta., .delta..sub.1 and .GAMMA. phases are all non-stoichiometric compounds, each having a certain range of Fe content. Although depending upon the alloying conditions applied and the type of base steel to be coating, this gives rise to changes in the contents of Fe of the .zeta., .delta..sub.1 and .GAMMA. phases and, in turn, causes the average content of Fe to vary throughout the coating layer, even though the .zeta., .delta..sub.1 and .GAMMA. phases are formed in the same quantities. Thus, it is unfeasible to provide an accurate determination of the average content of Fe in a coating layer by X-ray diffractometry.
As explained above, the X-ray diffractometric methods may be effective for an on-line, non-destructive and continuous determination of the degree of alloying. With the conventional methods wherein, as stated above, the diffracted X-ray detector is fixedly located at the 2.theta. position of the specific lattice plane of interest, however, it is impossible to measure the X-ray diffraction intensity constantly at the apex position of the X-ray diffraction peak, only to reduce the accuracy of measurement, and to get information of amounts of formation of the .zeta., .delta..sub.1 and .GAMMA. phases or the structure of the coating layer.
An accurate determination of an X-ray diffraction intensity at the apex location of an X-ray diffraction peak may be achieved by measuring the associated X-ray diffraction profile. To this end, the X-ray diffraction profile is conventionally measured by scanning one detector with respect to an X-ray tube within a certain 2.theta. range according to the .theta.-2.theta. scanning method. However, if it is intended to make an on-line determination of the X-ray diffraction, much time is then needed for detector scanning, during which the degree of alloying will vary, making it unable to measure the X-ray diffraction profile correctly.
The present invention seeks to solve the above-mentioned problems of the prior art by the provision of a system for making an on-line, non-destructive, continuous and accurate determination of the degree of alloying in galvannealed steel sheets.
When galvanized steel sheets are heat-treated for alloying, the .zeta., .delta..sub.1 and .GAMMA. phases are successively formed in the coating layer, as already referred to. And with the progress of alloying, the .zeta. phase disappears but, instead, the .GAMMA. phase grows thickly. These phenomena are illustrated in FIG. 2 with reference to the X-ray diffraction profiles measured by X-ray diffractometry, and are schematically shown in FIG. 3 as well. The three curves shown in FIG. 2 represents the X-ray diffraction profiles of galvannealed coating, each having a coating weight about 45 g/m.sup.2, which have been heat-treated on an salt bath for alloying. The curve 3 stands for the X-ray diffraction profile of the galvannealed coating which has been heat-treated at 500.degree. c. for 5 seconds for alloying, and the structure of the coating layer thereof is schematically sketched in FIG. 3a. The curve 4 explains the X-ray diffraction profile of the galvannealed coating which has been heat-treated at 500.degree. c. for 30 seconds for alloying, and the structure of the coating layer thereof is schematically depicted in FIG. 3b. The curve 5 shows the X-ray diffraction profile of the galvannealed coating which has been heat-treated at 500.degree. c. for 60 seconds for alloying, and the structure of the coating layer thereof is schematically sketched in FIG. 3c. With further reference to FIGS. 2 and 3, the longer the heat-treatment time and the more the degree of alloying, the smaller the volumetric proportion of the .zeta. phase and the lower the X-ray diffraction intensity. By contrast, however, an increase in the volumetric porportion of the .GAMMA. phase results in an increase in the X-ray diffraction intensity. The X-ray diffraction intensity of the .delta..sub.1 phase increases, as the heat-treatment time increases from 5 seconds to 30 seconds, but decreases from 60 seconds after the heat-treatment. This is because with the progress of alloying. The volumetric proportion of the .zeta. phase decreases with an increase in the volumetric proportion of the .delta..sub.1 phase; however, the continued progress of alloying causes a decrease in the volumetric proportion of the .delta..sub.1 phase due to an increase in the volumetric proportion of the .GAMMA. phase.
In addition, as the degree of alloying increases with an increase in the heat-treating time, the apex locations of the X-ray diffraction peaks of all the .zeta., .delta..sub.1 and .GAMMA. phases are shifted toward somewhat higher 2.theta. position.